ppc stxsibx and stxsihx instructions write too much data

ppc stxsibx and stxsihx instructions write too much data

stxsibx (Store VSX Scalar as Integer Byte Indexed X-form) is implemented
by first reading a whole word, merging in the new byte, and then writing
out the whole word. Causing memcheck to warn when the destination might
have room for less than 8 bytes.

The stxsihx (Store VSX Scalar as Integer Halfword Indexed X-form)
instruction does something similar reading and then writing a full
word instead of a half word.

The code can be simplified (and made more correct) by storing the byte
(or half-word) directly, IRStmt_Store seems fine to store byte or half
word sized data, and so seems the ppc backend.

https://bugs.kde.org/show_bug.cgi?id=430354